The present invention relates to a novel MFI-type zeolite and to a process for preparing said zeolite.
As a result of their geometrical selectivity and ion exchange properties, zeolites are industrially used on a wide scale, both in adsorption (e.g. gas drying, separation of straight and branched-chain paraffins) and in catalysis (e.g. catalytic cracking, hydrocracking, isomerization and oligomerization).
All the MFI-type zeolites containing Al and/or Ga prepared up to now have been synthesized either in an alkaline medium (pH generally above 9) in the presence or absence of organic agents, or in a fluoride medium (pH generally below 10) in the presence of organic agents.
The fluoride synthesis medium can be used for obtaining the MFI zeolite (FR-A-2567868, 2607128). All the syntheses of MFI zeolites in a fluoride medium according to the prior art are characterized by the presence in the reaction medium of an organic agent, e.g. tetrapropyl ammonium (TPA.sup.+) or tetrapropyl phosphonium. The proton form of the zeolite is then obtained by calcination. From an economic standpoint, it would be advantageous to be able to synthesize the MFI zeolite in the absence of an organic structuring agent. Compared with the alkaline synthesis medium (OH.sup.-) the fluoride medium has a certain number of very significant advantages (cf. e.g. J. L. Guth, H. Kessler and R. Wey, Proc. 7th Int. Zeolite Conf., Tokyo, Aug. 17-22, 1986, p.121). Thus, in the alkaline medium, most synthesized zeolites are metastable, so that during synthesis more stable solid phases may appear and undesired phases may be precipitated. This difficulty is increased when the quantities to be prepared increase, i.e. on passing from the laboratory to the industrial scale.
A supplementary advantage of syntheses carried out in a fluoride medium compared with those carried out in a conventional OH.sup.- medium is that solids are obtained, whose acid and ion exchange properties are of different natures. The acid catalysts prepared from the solids obtained in the fluoride medium have improved catalytic properties. At this level it is very important to point out that the crystallographic structure of a solid is not sufficient to entirely define its properties and more particularly its acid properties, which play a vital part in catalysis.